Free chlorine from dissolved chlorine gas is the most common disinfectant used in drinking water due to its high oxidation capacity.
The residual chlorine concentration is typically accurately controlled in a range around 0.5-2 mg/L to avoid both bacterial contamination (free chlorine <0.5 mg/L)1 and hazard to human health (free chlorine >2 mg/L)2.
Current standard technology for free chlorine sensing uses reagents which restricts its use to laboratory-based settings3. Therefore, free chlorine concentration is typically monitored only at the source of tap water supply. However, monitoring the concentration at many points along the water distribution network, continuously, is useful to guarantee drinking water quality as the free chlorine concentration can be affected by many parameters such as temperature, sunlight and time during water transportation2.
Known methods for measuring chlorine concentration include, for example, titration (iodometric or amperometric3), chemiluminescence and electrochemical methods. Titration-based approaches use reagents which are not suited for continuous or autonomous monitoring. The chemiluminescence method also uses reagents, where the sample is first reacted with chemiluminescent indicators to generate optical signal intensity which is proportional to the concentration of free chlorine in the sample4-6. In addition, known chemiluminescent methods use optical light sources and detectors, making it expensive7. Electrochemical methods, on the other hand, can be simple in design, do not need additional reactants, and produce sensory signals in an electrical form which is useful for autonomous, continuous monitoring8-10. Nevertheless, there are still some common drawbacks of known electrochemical sensors; for example, the sensing results are strongly affected by the flow rate and aging of the electrodes9,11; thus, frequent calibration is used.
The controlled modification of the band gaps of single wall carbon nanotubes (SWCNTs) has been applied in various electronic applications12.
The amphoteric nature of SWCNTs makes it feasible to modify the electronic properties of SWCNTs by doping with either noncarbon atoms or compounds at low concentrations13-16.